1. Field of the Invention
The present invention relates to protected documents of the color copy resistant type wherein a cancellation phrase appears substantially hidden to the human eye on the original documents, but, which is readily apparent to the casual observer on color copies made of the original.
2. Prior Art
Present protected documents include a cancellation phrase (VOID) of a single tone (includes percent of area covered by dots or other marks and the number of the dots or marks per inch) positioned precisely into a background tone composed of a set of dots or marks significantly different in size and number per inch from that used for the cancellation phrase. Dot sizes and number of lines per inch may be made up of several different combinations such as:
1. Void dots about 0.010 diameter with 65 lines per inch coupled with a background dot of about 0.005 diameter with 130 lines per inch.
Transmission densitometer readings in production grade negatives may be show about 0.65 units for the 0.005 diameter dots.
2. Other combinations such as 62.5 lines per inch for the cancellation phrase and 125 lines per inch for the background also may prove useful. In addition to the dot size combinations set forth in No. 1 above, other variations may also prove useful.
Suitably combined sets of cancellation phrase dots and background dots have been successfully camouflaged by another patterned screen exposed in combination with the phrase and the background screen. Depending on procedure, the resultant photographic film will have dots or marks removed from the phrase and the background or base dots enlarged in the phrase and in the background. This combined film can be used to make printing plates or photographic film copies for distribution to various printing operations.
The above-described system gives good protection against copying to suitably printed documents when copies are made at normal copier settings. However, protection is not complete over the full copier range.
It has been recognized that different dot size pairs (e.g., 65 line, 0.010 diameter and 130 line, 0.005 diameter) have greater or lesser ability to emphasize the cancellation phrase above the background when copied at lighter or darker copier settings.
Efforts to develop a combination of more than a single screen pair have proved aesthetically unsatisfactory despite the fact that the effective range was increased.
The unhappy approach took the form of blocks or bands with one pair of screens per block or band.
All of the cancellation phrases could be camouflaged successfully but the bands or blocks remained and rendered the document unsightly because of the obtrusive background pattern.
The present invention provides a means of combining two or more significantly different background and phrase combinations into a single area on the document thereby avoiding the obtrusive patterns which inevitably result from previous approaches.